A common mechanism for converting linear motion to quarter-turn rotary motion is the Scotch yoke actuator. A Scotch yoke actuator provides a rotation of approximately 90 degrees, with a torque that is higher at the ends of travel and lower in the middle. This is beneficial for operating valves that have higher torque requirements at the fully closed position. Also, Scotch yoke actuators are generally less costly to manufacture than other actuator designs when a torque output of greater than 15,000 inch-pounds is required, making the Scotch yoke actuator generally the design of choice for operating large valves.
In a typical Scotch yoke design, a push rod moves back and forth past a rotatable shaft that is offset from the rod and perpendicular to it. The push rod is coupled to the shaft by a lever arm or yoke that converts the linear movement of the rod to a rotary movement of the shaft, with a maximum rotation angle of approximately 90 degrees. The push rod is coupled to the yoke by a yoke pin that passes through a hole in the push rod and a slot in the yoke, thereby allowing the yoke pin to slide along the slot in the yoke as the push rod moves and the yoke and shaft rotate.
A significant disadvantage of the Scotch yoke design is that during most of its 90 degree rotation the force applied by the push rod to the yoke is not perpendicular to the axis of the yoke (the axis of the yoke being defined as an imaginary line that joins the shaft to the yoke pin). This causes “side loading” effects, which are reactionary forces applied perpendicular to the push rod and to the rotatable shaft. The side loading effects must be counteracted so as to prevent the push rod and the rotatable shaft from being moved out of alignment.
In many Scotch yoke designs, push rod support bushings are used to counteract the side loading effect and maintain the orientation of the push rod. But significant side loading force combined with constant sliding of the push rod past the support bushings can cause the bushings to wear and eventually to fail. Also, the side loading force can be transmitted past the support bushings by a lever effect and can damage the piston, spring, or other mechanism that drives the push rod. In addition, the friction between the support bushings and the push rod can cause “jump starts” and uneven movement of the push rod, making the design unsuitable for applications such as valve throttles that require smooth, controllable movement.
In some Scotch yoke actuators, instead of using push rod support bearings, the yoke pin extends beyond the yoke and is supported on either side by slots in a housing that surrounds the yoke, thereby counteracting the side loading effect on the push rod. This approach avoids the wear, unsmooth movement, and other problems associated with push rod support bushings, but does not eliminate side loading of the rotatable shaft, which can cause wearing and failing of the bushings or other structures that support the shaft.
Scotch yoke actuators require that the yoke pin remain parallel to the axis of the rotatable shaft. In the traditional design, a split yoke surrounds the push rod, with the yoke pin passing through the push rod in the middle and yoke slots on either side. In this design, the hole through the push rod is responsible for holding the yoke pin in proper alignment with the rotating axis. Any tendency of the push rod to rotate can cause misalignment of the pin, and even failure of the coupling. Even if slots in a yoke housing are used to maintain the orientation of the yoke pin, rotational tendencies of the push rod can cause wear to the yoke pin and to the yoke pin hole.